Well I finally reached the secret.
Enough of this game for me :-)
I definitely wont do a new game with the unlocked content all over again :-)
general tip:
After a ton of time and having it run over night, i had about 8 millions coins.
first I bought all the balls (light and normal in that order) that I could.
After that I would jsut switch between normal and boss fighting mode and from time to time buying the +1% upgrade and buying new balls, never prestiging again.
still takes some time to beat all the levels (gotta keep in mind before you autofight to have some money collected in the balance, as the autofight uses said money with each defeated monster. so having almost no money will stop the autofight after like a few bosses